The invention relates to optical fiber fabrication, and more particularly to crack detection in sol-gel bodies.
Sol-gel tubes are formed as precursors to optical fibers. The sol-gel tubes are typically formed by extruding gel from a mold under water, then drying them to remove unwanted water. Nitrogen is then flowed around the body to extract organics and other undesirable materials. Process temperatures may reach higher than 1000xc2x0 C. These high temperatures limit in situ monitoring of sol-gel tubes during the formation process. This is problematic because the sol-gel tube formation process typically spans a 48 hour period, making it difficult to isolate a process step or steps during which problems such as cracking occur.
Cracking is common during sol-gel tube fabrication. Acoustic emission techniques have been used to detect cracks in tubes. The techniques comprise monitoring sound waves generated by the cracks using contact sensors. For example, a microphone may be placed against a tube to pick up sound generated by a crack. Use of contact sensors is limited to process steps wherein temperatures are not too high. This eliminates their use in process steps in which cracking is most likely to occur. Accordingly, there is a need for a crack detection method and apparatus that may be used during high temperature process steps so that cracks can be detected as they occur. This may be beneficial to sol-gel body fabrication process development, as it allows pinpointing of process steps in need of process parameter modification and fine-tuning.
A crack detection method is disclosed capable of detecting cracks of about 5 xcexcm wide or greater. The method is useful for detecting cracks in sol-gel tubes. A signal is transmitted from a source to a body in which cracking may occur and is reflected back to a receiver. An output voltage from the sensor is measured which is proportional to a signal characteristic change between the transmitted and received signals. Signal characteristics may include one or more of the following: frequency, phase and amplitude. The signal may result from one or more mechanisms selected from the group consisting of, microscopic motion of the body such as occurs during cracking, a media transition (solid, liquid, gas) as would occur when a tube has cracked, and solvent movement, such as a release during a fabrication process drying step.
Further disclosed is a crack detector wherein cracks are detected according to the method described above.